1. Field of the Invention
The present invention relates to a semiconductor device and a touch sensor, and particularly to a semiconductor device for determining whether or not a touch electrode is touched by a human body as well as a touch sensor using the semiconductor device.
2. Description of the Background Art
Touch sensors that determine whether or not a touch electrode is touched by a human body, based on a change in capacitance of the touch electrode, are becoming widespread in these years. Conventional touch sensors are each configured to include a plurality of touch electrodes, two capacitors and a resistor element provided for each touch electrode, an IC (Integrated Circuit) for use in the touch sensor (hereinafter “touch sensor IC”), and a general-purpose microcomputer (see for example Japanese Patent Laying-Open No. 2006-078292).
Japanese Patent Laying-Open No. 2009-042903 discloses a capacitive sensing apparatus configured as follows. Specifically, the capacitive sensing apparatus is equipped with a display apparatus and manipulated according to a display. Specifically the capacitive sensing apparatus includes display means for producing the display, a transparent sensing electrode laid on a display surface of the display means, charging and discharging means for repeatedly charging and discharging the sensing electrode, power storage means changing the voltage according to a charge supplied to and released from the sensing electrode through repeated charging and discharging of the sensing electrode, determination means for making a determination as to whether a value that changes according to a change in voltage of the power storage means has reached a threshold, capacitance calculation means for calculating a capacitance of the sensing electrode based on the number of times the charging and discharge means charges and discharges before the determination is made by the determination means, and control means for controlling the whole state and operation. The determination means makes a comparison between the value that changes according to a change in voltage of the power storage means and the threshold, each time the charging and discharging means charges and discharges. When the value is equal to or more than the threshold in two or more successive comparisons, the determination means determines that the threshold has been reached.
The conventional touch sensor having two capacitors and a resistor element for each of a plurality of touch electrodes, however, has a problem that a large number of externally-mounted components is required for the touch sensor IC. An increased number of externally-mounted components leads to an increased size of the touch sensor and increased cost. Further, because of a greater influence of difference between the externally-mounted components, malfunction is more likely to occur. In addition, an increased number of external terminals of the touch sensor IC is required for connecting the externally-mounted components.
In the conventional touch sensor, a CPU of the microcomputer executes software (program) to control the touch sensor IC and determine whether or not a touch electrode is touched by a human body. Then, another problem arises that the microcomputer has to bear a heavy load, resulting in increased power consumption.
As for Japanese Patent Laying-Open No. 2006-078292, a measurement of voltage is taken per charging/discharging. In the case where noise is generated in the period in which the measurement is taken, the measurement as taken could be in error depending on the intensity of the noise. As for Japanese Patent Laying-Open No. 2009-042903, a plurality of measurement periods is provided per charging/discharging and a measurement of voltage is taken in each measurement period. In the case where noise is generated in this measurement period, it is highly possible that the measurement as taken is in error depending on the characteristics of the noise. Japanese Patent Laying-Open No. 2009-042903 discloses that multiple measurements are taken with the aim of reducing noise from the display apparatus. The noise from the display apparatus has a relatively high frequency, and Japanese Patent Laying-Open No. 2009-042903 is based on the assumption that noise will be detected in a relatively small number of measurement periods among the multiple measurement periods. In contrast, when a touch electrode is touched by a human body and noise of a relatively low frequency arises from the human body, the noise is detected in a greater number of measurement periods among the multiple measurement periods.